This particular clip requires a lot of force to be used to remove the door trim as well as when inserting it back into the door before reattaching the trim. It has a thin piece on each side and one side broke previously. Can't remember how that happened but I could have been pulling it in the wrong direction as it is very difficult to remove, especially the first time. Luckily the rear door trims don't have this clip and come off easier. Have you seen this clip before? It does seem difficult to identify online and is not included in any of those sets that include 100's of different clips.
I used STP Gold which is a Russian Dynamat equivalent. Dynamat is extremely expensive here but there are some local copies which are extremely cheap that I could have used. STP was somewhere in the middle so seemed like a sensible choice. It's easy to cut and seems to work well, but I have no experience with other brands.
